This invention relates to pressure operated switches and more particularly to a switch that is resistant to fire.
Pressure switches are widely used to monitor the pressure conditions of various types of fluid systems. For example, it is common to equip fluid systems with pressure responsive electrical switches that are actuated in the event of an undue change in the fluid pressure. Although switches of this type function well for the most part, they are not suited for high temperature service or use in environments which may be exposed to the possibility of fire. Typically, the pressure sensing function is performed by a force balanced piston assembly that is sealed from the fluid medium by a flexible diaphragm and a conventional O-ring. When exposed to high temperatures such as occur during a fire, these and associated components of the switch either burn or melt, and the fluid medium can escape to the surrounding area through the damaged switch device. If the fluid medium is toxic or flammable, its release can lead to serious adverse consequences.
Accordingly, it is the primary object of the present invention to provide a pressure responsive switch that is constructed in a manner to contain the fluid medium even under fire conditions when less heat resistant components have burned or melted. In accordance with the invention, the diaphragm which is directly exposed to the fluid medium is constructed of corrosion resistant steel film and is backed by an elastomer disc and an annular backing ring which is also formed of corrosion resistant steel. The diaphragm is welded to the housing of the unit and to the backing ring by a pressure tight weld. The result is that the fluid medium is contained by a welded assembly of corrosion resistant steel components that does not break down even when subjected to the high temperatures that occur during a fire.